Simplify the following expression: $r = \dfrac{n - 3}{3n} \div \dfrac{1}{4}$
Explanation: Dividing by a number is the same as multiplying by its inverse. $r = \dfrac{n - 3}{3n} \times \dfrac{4}{1}$ When multiplying fractions, we multiply the numerators and the denominators. $r = \dfrac{(n - 3) \times 4} {(3n) \times 1}$ $r = \dfrac{4n - 12}{3n}$